68k (and the NuBus PPC) PowerBooks maxed out at 1.5MB/s because of all of the legacy chips they used to reduce cost and maximize compatibility with the Duo System devices. However there were a number of machines that used 2.5" SCSI drives and also supported fast SCSI, such as portable computers from IBM and Tadpole, a few low-cost workstations from DEC, and some high-end digital audio samplers.
I've never seen an ATA-to-SCSI adapter board in a 5x0-series machine, though I did find one in a 1x0 series before (a 160 of some sort, I think). At the very least, IBM and Toshiba built 2.5" SCSI drives up to 1.2GB capacities, so it wasn't really required to use an adapter to get high-capacity drives in 1994 unless you were buying some tail-end refurbs 1996 and there was no stock of SCSI drives remaining, or a dealer had unsold inventory and upgraded them to try to get them sold. I'd suggest using IBM's 1.2GB DPRS drives (SCSI versions of the DPRA drives that shipped in the Duo 2300 and PB 5300) but I found out they don't have internal terminators so they don't work well in a PowerBook unless you have an external device (CDROM, MO, HD, Zip, etc) providing active termination. Really annoying.